Congenital diaphragmatic hernia (CDH) is a developmental defect of the diaphragm, allowing herniation of abdominal viscera into the chest. It occurs in 1 in 2,000 to 3,000 live births (1). Although the postnatal treatment has become more standardized, substantial morbidity and mortality result from the associated pulmonary hypoplasia and abnormal vascular development of the newborn. A subset of fetuses with liver herniation and a smaller lung size, represented by an observed over expected lung-to-head ratio (O/E LHR) under 25%, have higher mortality and morbidity rates and currently are offered in utero fetal surgery. Fetoscopic endoluminal tracheal occlusion (FETO) (2) prevents normal egress of airway fluid, which in turn induces tissue stretch, acting as a signal for lung growth. A lack of understanding of the molecular mechanisms underlying pulmonary hypoplasia in CDH hampers progress for potential in utero therapies and case selection. A common genetic cause for CDH is unknown (7). However, it is widely accepted that the diaphragmatic defect and pulmonary hypoplasia result from a shared developmental insult (8, 9).
MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression through post-transcriptional silencing of messenger RNAs (10). MicroRNAs are essential for normal organogenesis during embryonic development. For example, targeted deletion of miR-1-2 leads to congenital heart defects in mice (11). Previous studies have identified differential miRNA expression between various stages of lung development, but these studies did not provide much functional information (12). Whether specific miRNAs play a role in the pathogenesis of human congenital lung diseases remains unknown. Isolated CDH is characterized by abnormal lung development and may therefore serve as a template to study molecular mechanisms driving lung growth and differentiation. The prenatal period offers a unique clinical research opportunity, since CDH cases can now be well characterized by prenatal imaging and postnatal outcome.
Only about 15%-20% of individuals with CDH have been identified with chromosome abnormalities or a single gene disorder and in all these patients, CDH occurred with additional malformations (complex-CDH) (71, 72). The etiology of the remaining 80%-85% (isolated-CDH) is currently unknown. Little is known about the role of specific miRNAs during lung development and it appears that different groups of miRNAs can regulate different stages of lung development. Most research studies in lung morphology and organogenesis are limited in comparing expression profiles of miRNAs at various stages of lung development (75). The miR-200 family is comprised of five members which are transcribed in two clusters: miR-200b, 200a and 429 share a common transcription start site on chromosome 1, while miR-200c and 141 are transcribed as a single unit from chromosome 12(76). MiR-200 family members are considered as epithelial markers and can revert an epithelial-mesenchymal transition (EMT) in cancer and pulmonary fibrosis by directly targeting two transcription factors, ZEB1 and ZEB2(77, 32, 78, 79). However, the functional role of miR-200b in normal lung development and hypoplastic lung resulting from CDH is currently unknown.
The elegant nitrofen rat model has been widely used to investigate the patho-physiological mechanisms of abnormal lung development associated with CDH. Moreover, it has direct resemblance because of its striking similarities to human CDH, such as liver herniation, a large diaphragmatic defect, and asymmetrical lung hypoplasia and pulmonary hypertension (70).